The present invention relates to an encoder and method for precisely indicating the position of a first member with respect to a second member. The invention is particularly useful for optical encoders, and is therefore described below with respect to such an application, but it will be appreciated that it could also be used with respect to non-optical encoders, such as brush-and-slipring encoders and magnetic encoders.
An encoder of the common type is an electro-mechanical device coupled to a moving member, for example a rotating shaft or a linearly sliding member, that produce an electronic signal which can be processed to provide information regarding the position of the moving member, i.e. the rotation angle of the shaft, or the linear position of the sliding member.
Encoders may be of the incremental or absolute type. An incremental encoder provides information about the rotation angle relative to the initial position, i.e. the position of the moving member at the time the encoder was activated. Absolute encoders give absolute position information, not depending on the initial position at activation time.
Absolute rotary encoders can be classified into single turn absolute and multi-turn absolute rotary encoders. Single turn absolute rotary encoders provide shaft angle information modulo one turn, while multi-turn rotary absolute encoders provide shaft angle information including the number of turns executed by the shaft, starting at a time origin such as encoder manufacturing time.
Similarly, absolute linear encoders of a first type provide position information modulo a given linear period. This kind of linear encoder is used for example with linear motors, in order to provide motor magnetic pole position information relative to the moving coils, allowing brushless commutation by the motor controller. A second type of absolute linear encoder provides position information relative to a constant absolute origin.
Typically, in a single turn absolute rotary encoder of the prior art, a pattern of circular tracks consisting of sections with different optical properties is applied on a disc fixed to a rotating shaft. Such optical property may be the transparency (light transmissivity or light non-transmissivity) of a section. An example of a prior art disc is shown in FIG. 1, whereas each circular track on the disc is provided with an optical sensor and is scanned by light rays such that each sensor will output a signal representing the optical property of the section of pattern facing it at a given point of time.
The signal output by the sensor is a binary “0” or “1” signal (bit). These bit signals are combined to create a binary code which is dependent on the rotation angle of the disc, and the code is then converted to position information according to a predefined table.
Typically, special patterns giving rise to special types of code (Gray code) are used. As known in the art, the term Gray code refers to a code for which only one bit of the code is changed between two consecutive increments of the encoder disc. This type of code prevents the creation of an erroneous code at transitions between increments.
A disadvantage of prior art encoders of the type described above is that several concentric tracks are required for providing information regarding the position of the moving member. Since each track requires a minimum area for reasonable precision, the total size of the disc is undesirably large. For example, the disc pattern shown in FIG. 1 has eight concentric tracks and the track with the smallest diameter must have the same angular precision as the one with the largest diameter, such that dimension tolerance becomes very low. Consequently in the prior art encoders, the minimum possible encoder disc size is limited, whereby manufacturing costs are increased.
A number of attempts have been made to reduce the minimum number of tracks. Patent Application Publication EP 0-332-244-A1 shows an arrangement where only two tracks are used, a coding track and a so called “Clock” track. However, this patent still requires two tracks, and the code created is not a Gray code, i.e. for two adjacent increments, there are positions where more than one bit of the “clock” track is changed.
PCT WO 91/00984 by Nagase and Higashi describes a one track absolute code system. However this patent requires that a number “n” of sensors be placed at “n” consecutive increments. The sensors then must be very small, so that they can be placed next to each other. This requires the manufacturing of a specially designed opto-electronic chip. Such a chip must be designed for each particular encoder size, and thus cannot be used for several types of encoders of different sizes.
Another known technique employs a single track with three sensors that provides six codes for six increments. This technique is widely used with brushless servo motors to retrieve position information about the rotor pole in which case Hall sensors are commonly used to sense the magnetic poles of the rotor. However this technique is only applicable with three sensors, and position information resolution has but six values.